This application claims the priority of German patent document 198 13 945.4, filed Mar. 28, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a control device for influencing the driving dynamics of a four-wheel vehicle.
A device for controlling four-wheel steering of a motor vehicle disclosed in German patent document DE 41 39 009 C1 includes positioning devices for the front wheels and the rear wheels, and generates control signals that actuate the positioning devices in accordance with specified steering laws. The specified steering laws are intended to permit control of the yaw and transverse acceleration behavior of the vehicle, in order to achieve an optimum response behavior relative to slip angle development when steering.
This control device has the disadvantage that the steering engagement with the rear wheels requires a high cost of construction, whose advantages are usually realized only in driving dynamic marginal situations. Deliberately influencing the driving behavior below the marginal range, especially a separate control of yaw and transverse acceleration behavior, is not possible however.
The object of the invention is to set and tune the transverse dynamic properties of a vehicle in the normal driving range at a low cost of construction.
This and other objects and advantages are achieved by the control device according to the invention, which makes it possible to produce changes in the trail angle at the front wheels and rear wheels to produce a specific desired dynamic driving behavior. The driving behavior relative to the transverse dynamics of the vehicle can be specified in accordance with objective criteria for vehicle evaluation and the trail angle at the front and rear wheels can be adjusted on the basis of these criteria.
The specified target values which are used as criteria of vehicle evaluation are the steering ratio, the self-steering gradient, and the float angle gradient, which are supplied as set signals to the control device. From these target values, the control device, on the basis of the current vehicle speed and steering angle, as well as other vehicle-specific parameters, generates control signals by which the positioning devices at the front and rear wheels are actuated. The control signals, which represent additional trail angles for influencing the dynamic driving behavior, are determined relative to the trail angles of the basic tuning and are added to the latter. The vehicle speed and the steering angle can be detected by sensors.
The steering ratio, the self-steering gradient, and the float angle gradient as setpoints to be specified are selected specifically for the vehicle and in accordance with desired driving behavior. On the basis of experiential values, for example for setting a "sporty" driving behavior of the vehicle, a small float angle gradient can be specified, by which the yaw movements of the vehicle are minimized.
The trail angles to be determined are available as steady-state control signals by which the trail angle can be adjusted for the front wheels and the rear wheels during driving as a function of the steering angle and the vehicle speed. The adjustment of the steady-state trail angle alone influences the dynamic driving behavior with respect to the yaw rate and the transverse acceleration of the vehicle.
In addition to the steady-state adjustment, according to an advantageous improvement, the trail angle can be dynamically tuned during driving with control signals variable as a function of time. In dynamic tuning, the steady-state additional trail angle is rendered dynamic, so that the additional trail angle follows a time-dependent transient characteristic. Preferably, a relationship is created between the time-variable curve of the additional trail angle and the time-variable curve of the front and rear additional float angle and of an additional self-steering angle that has been rendered dynamic. The dynamics are expressed with the aid of a Laplace transform, with the dynamic terms being dropped in a steady-state curve and the relationships being identical for the steady-state and dynamic additional trail angles.
The curve of the frequency response of the transverse acceleration can be influenced by a transverse acceleration amplification factor that can be specified in advance, especially in the range between 0 and approximately 2 Hz. If the amplification factor is chosen to be larger than zero, a driving-dynamic critical drop in the transverse acceleration amplification in this frequency range can be avoided. If the amplification factor is chosen to be less than zero, the transverse acceleration amplification can be reduced.
In addition, by a suitable choice of the factors that influence the dynamics, the yaw rate frequency response can remain unchanged, and thus retain the same curve as in basic tuning, without applying a steady-state or dynamic additional trail angle. Changes in the frequency response of the yaw rate, caused by a time-variable curve of the additional float angle, can be compensated.
The yaw frequency response in a preferred embodiment can be influenced directly by specifying the yaw natural frequency and yaw damping.
Dynamic tuning of the additional trail angle is possible independently of the steady-state setting. In a first expansion stage, adjustments can be made initially only in the steady state, while tuning can then be performed dynamically in the second expansion stage.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.